314 THE MICROSCOPE AND ITS REVELATIONS. 



rior of the earth. Again, we are often enabled by the same means to 

 trace-out the ' metamorphic ' action by which one kind of rock has been 

 converted into another subsequently to its first deposition as a sediment. 

 Of this the change of a calcareous deposit made-up of the remains of 

 Foraminifera with fragments of shells, corals, etc., into a crystalline 

 Limestone, is one of the most common; occurring wherever the rock has 

 been subjected to pressure and contortion, and especially in the near 

 neighborhood of igneous outbursts. And there can now be little hesita- 

 tion in attributing much of this conversion to the solvent action of water 

 raised to a very high temperature under enormous pressure. A very 

 curious piece of evidence, moreover, has now been furnished by Micro- 

 scopic study, in support of the doctrine which other considerations render 

 probable, that some forms of Granite (to say the least) have been gen- 

 erated from sedimentary rocks by metamorphic agency of a like nature. 

 For it has been shown by Mr. Sorby that the quartz-crystals of Granite 

 often inclose water or other liquids (sometimes liquid carbonic acid) in 

 cavities in their interior; which cavities, however, are not filled with the 

 liquid, the remaining spaces being occupied by vapor. This fact cannot 

 be otherwise accounted for, than by supposing that the crystallization must 

 have taken place in the presence of water; and that this water, though 

 liquid, must have been so hot as at that time to fill the cavities which it 

 now occupies only partially, the size of the present vacuity marking the 

 amount of its subsequent shrinkage during the cooling of the mass. 



708. As this study, however, can only be successfully prosecuted by 

 such as have previously obtained a considerable knowledge of Mineralogy, 

 further details would obviously be unsuitable to our present purpose; 

 which is only to excite an interest in these researches, and to give such 

 general directions as will be of service to beginners who may be disposed 

 to follow them out. The mode in which Rock -sections are to be cut, is 

 essentially the same as that for which directions have already been given 

 ( 192-196); but it will be found desirable to use broader and thicker 

 glasses than the ordinary 3x1 inch size, so that the sections may be 

 about an inch square. The emery-plate should only be used for the 

 hardest rocks, as the softer will be disintegrated when rubbed upon it. 

 For these last, a fine corundum-file, or a piece of pumice-stone, is to be 

 preferred in the first instance, and a fine Water-of-Air stone for finish- 

 ing. When the rock is very friable, it may be saturated with hardened 

 Canada balsam before rubbing down. As sections of the thinness usually 

 required may not bear being transferred from the glasses to which they 

 are cemented, it will be desirable that the attachment of a flattened and 

 polished surface to the glass on which any section is to remain, should be 

 finally made before the reduction of its thickness has been such as to in- 

 volve the risk of its fracture in the process. 1 



1 An "Elementary Text-book of Petrology" has lately been published by Mr. 

 F Rutley, of EL M. Geological Survey. The more advanced Student should have 

 recourse to the successive Memoirs published by Mr. Sorby in the Journal of the 

 Geological Society, the Proceedings of the Yorkshire Geological Society and else- 

 where, especially the following: ' On some Peculiarities in the Microscopic Struc- 

 ture of Crystals,' in " Journ. of Geolog. Society," Vol. xiv., p. 242; 'On the Mi- 

 croscopic Structure of Crystals, indicating the Origin of Minerals and Rocks,' Op. 

 cit., p. 453; 4 On the Original Nature and subsequent Alteration of Mica-Schist/ 

 Op. cit., Vol. xix., p. 401; 'Sur 1' Application du Microscope a 1'Etude de la 

 Geologie Physique, in "Bull. Soc. Geol. de Paris," 1859-60, p. 568; and his Presi- 

 dential Addresses to the Geological Society, 1879 and 1880. Also the Memoir by 

 Mr. David Forbes, ' The Microscope in Geology,' in the "Popular Science Review," 



